I. Field of the Invention
The present invention relates to a detection circuit of the current in an MOS type power transistor.
II. Prior Art and Other Considerations
In such a device it is necessary to control the output current, e.g., during overload or shortcircuit situations, so as not to destroy the device itself.
It is also necessary to provide for the possibility of having a diagnostic signal when the output current is lower than a pre-set minimum value, e.g., because the output load is disconnected, or of having a diagnostic signal when the output current is higher than a pre-set maximum value.
On the basis of the prior art the control of the output current in an MOS type power transistor exploits the `current detection` technique to ensure a minimum power dissipation and a minimum voltage drop across the power device.
In the cases wherein the power device consists of n cells, the current in each individual cell is 1/n of the total. With the `current detection` technique this current may be used to control the current flowing through the entire MOS power device.
A known circuit embodiment for limiting the current flowing through a power transistor according to the `current detection` technique provides for the use of a detection transistor connected with drain and gate in common to the power transistor and having characteristics such that the current flowing through it is equal to a fraction of the latter's current and of a control circuit which operates in relation to the voltage which is established at its input due to the current flowing through said detection transistor.
The control circuit, according to the known art, comprises a pair of comparators having inputs connected together, with a series of a resistance and of a voltage generator connected between them. As the current flowing through the power transistor and thus through the detection transistor rises, the voltage across the resistance increases until it equals that of the voltage generator. The control circuit then becomes active and controls the common gate of the power transistor and of the detection transistor thus limiting the current flowing through them.
This circuit embodiment can also be used to signal when the current through the power transistor falls below a pre-set threshold value, and in particular to signal when the load is disconnected.
In this case the signal is executed through the output of a comparator one of whose inputs is kept at a constant threshold voltage and the other of whose inputs receives a voltage proportional to the current flowing through the power transistor.
This technique has several disadvantages such as the high number of components, the complexity of accomplishment and a comparatively low and barely controllable degree of accuracy.
The object of the present invention is to accomplish a detection circuit of the current in an MOS type power transistor which has a lower number of components and at the same time is accurate and hardly sensitive to variations in the parameters external to the circuit.